1. Field of the Invention
This invention relates to a radial pneumatic tire and more specifically to an improved self-supporting run-flat tire.
2. Description of the Prior Art
There is a current ongoing effort by automobile manufacturers to eliminate the spare tire in order to reduce vehicle curb weight, increase available space within the vehicle and provide operator convenience. This is particularly true for vehicles having higher comfort specifications such as conventional luxury, family or urban-economy type vehicles. For example, new generation electrical and hybrid type vehicles have critical space and weight restrictions.
One solution to increase trunk space and reduce weight of the spare tire is the mini-spare. This tire is a specifically designed and constructed narrow tire mounted on a special rim. The tire has a slightly smaller diameter than the standard factory equipment tire. These tires are very limited in the number of travel miles permitted and the speed of the vehicle. They are intended to get the vehicle to a service station so the standard tire can be repaired. Special mini-spare standards are provided by the Tire and Rim Association (T&RA) of Copley, Ohio and are based on vehicular weight and performance. The mini-spare is not the best solution because the tire loses air over time (by permeation) and will not have the performance characteristics of a properly inflated mini-spare. Furthermore, space for the removed flat tire is frequently not available.
Another recent solution is the run-flat tire. The advantage of this tire is to eliminate the need of a spare tire and ancillary equipment, achieving substantial savings in vehicle weight and increasing the space for other automotive systems and cargo. Numerous variations of run-flat tires have been developed. These involve changes to the structure of the tire itself and modifications to the rim to hold and support the flat tire. Each variation is limited by safety restrictions on vehicle speed, length of travel, zero inflation pressure handling and the magnitude of the lateral accelerations that force the bead of the tire off the rim seat. Further, the best solutions are those which do not affect the vehicle's nominal performance. Therefore, the need for improvements in the design of run-flat tires continues.
A number of generic features of run-flat tires have been disclosed which yield improvements, although limited, in vehicle performance. These features include thickened tire sidewalls, sidewall reinforcing plies, tire bead seat and vehicle rim configuration modifications, tire sidewall to rim flange contact and tire belt package edge modifications. Each of these features can be used to help solve known run-flat performance problems.
One feature of some run-flat tires is thickened sidewalls to support the vehicle after loss of inflation pressure. Such a sidewall, as the tire is viewed in cross-section, presents a crescent-shaped mass of rubber to the inside of the carcass reinforcing. On complete deflation of the tire, the crescent-shaped mass is put into compression while the carcass cord reinforcement is in tension; thereby preventing collapse of the sidewall. The respective inner wall surfaces of the tire do not contact one another and the rolling radius of the tire is maintained at a relatively large percentage of the inflated rolling radius of the tire. Seven patents that disclose a thick sidewall design are U.S. Pat. Nos. 4,067,374; 4,779,658; 5,058,646; and 5,217,549, European Patent No. 456,437 (EP), Japanese Patent No. 1-30809 (JP) and French Patent No. 2,469,297 (FR).
U.S. Pat. No. 4,067,374 discloses the use of a crescent-shaped sidewall reinforcing rubber portion inside the carcass layer which has a high dynamic modulus with low hysteresis properties and high aging properties. The crescent-shape sidewall reinforcing rubber is put into compression while the cords of the carcass are put into tension, thereby inhibiting collapse of the sidewall. A cap ply located outside the belt package cooperates with the sidewall construction to increase the run-flat performance of the tire. The patent also discloses a lower sidewall support on the tire that contacts a rim flange when the tire is in an uninflated mode.
In European Patent No. 458,437, the inner sidewalls of the tire have a crescent-shaped reinforcing rubber with a specified radius of curvature of the axially inner face in relation to the section height of the tire. Two radial carcass plies are turned up around the bead cores from the inside to the outside. Two cap plies are positioned radially outward of two belt plies. In addition, this run-flat tire design includes an extension of the bead area to form a bead toe for a bead retaining system.
The crescent-shaped reinforcement rubber portion of the run-flat tire in U.S. Pat. No. 4,779,658 has two layers; i.e., an anticrack layer adjacent to the carcass and a reinforcing layer inside the anticrack layer. The reinforcing layer provides more support for the tire when deflated and the anticrack layer is stated to provide a balance between ride comfort and crack resistance. This tire also has a protruding rubber bead toe portion supported by a rubber chafer and a fabric member made of textile cord.
U.S. Pat. No. 5,058,646 is similar to EP 458,437 but discloses a three-layer crescent-shaped cushion on the inner sidewall in terms of thickness of the layers. A hard rubber bead filler is further disclosed for the bead area of the tire.
The pair of crescent-shaped elastomeric reinforcing members disclosed in U.S. Pat. No. 5,217,549 are preferably for high profile tires having a section height of 5 inches (127 millimeters) or greater. Sidewall stiffness is achieved by a single high modulus crescent-shaped member in each sidewall with a reinforcing bias ply strip on the inside or the outside surface of the two carcass plies. These bias ply strips are bias at 60 degrees and reinforcing members are of a nylon material.
In the JP 1-30809 patent a crescent-shaped low heat generating rubber with a specified maximum thickness of 1.0 to 1.5 times the sidewall rubber plus the carcass thickness is arranged inside the sidewall. The crescent-shaped run-flat strip extends radially inward and overlaps the apex of the bead filler rubber by 10 millimeters or more.
A two part crescent-shaped sidewall reinforcing ply of FR 2,469,297 has a relatively thick mass. The exterior portion adjacent the carcass plies is of a flexible cellular structure having a relatively low density and Shore A hardness. Preferably the neutral axis of the sidewall during bending is located in the interior portion of the reinforcing ply.
A critical and limiting feature of the run-flat tire is the ability of the deflated tire to stay on the rim during cornering maneuvers of the vehicle. This is known in the art as resistance to bead-unseating or bead retention. Bead unseating resistance is improved by extending or recessing the toe portion of the bead area to engage a rim having an extended or recessed portion. This feature of run-flat tires is noted in European Patent 456,437 (EP), U.S. Pat. Nos. 4,554,960; 4,779,658; and 4,917,164 and Japanese Patent No. 2-179513 (JP).
In patent EP 456,437, each bead area has a relatively thin rubber portion shaped axially inward of the carcass to form a bead toe. The toe extends radially inward to be inserted into a rim groove at the axially inner end of a tapered bead seat of a wheel rim. The base of the bead of the tire is further provided with a groove immediately axially outside the bead toe and inside a bead core. The groove fits into a hump formed in the bead seat of the rim for which the tire is designed.
U.S. Pat. No. 4,779,658 also shows a protruding rubber member which serves to reinforce the bead area to prevent the bead from unseating during run-flat travel. The protruding rubber seats the tire to a rim which is modified to receive the protruding rubber member. U.S. Pat. No. 5,058,646 discloses a similar protruding member that seats in a modified rim.
The extended rubber toe portion of each bead area disclosed in U.S. Pat. No. 4,917,164 is a hard rubber member bonded to the crescent-shaped reinforcing layer. The toe portion has a preferred elastic modulus at 100 percent unit strain of 75 to 95 kilograms per square centimeter. A fabric reinforcing member or ply is attached to the exterior of the toe portion as an interface to the rim seat. Another rubber member is bonded to the heel portion at the rim interface. All these components help to maintain the tire on the rim after it becomes deflated.
The problem of maintaining the tire on the rim with a loss in inflation pressure is also discussed in U.S. Pat. No. 4,554,960. To resist bead-unseating, this patent discloses a specially designed bead area base and precise placing of the beads on the rim seats. A rim hump is formed on a standard rim with a circumferential hump having radially a cylindrical generatrix. Japanese Patent No. 2-179513 also discloses the modification of the bead toe portion as well as the rim seat.
With the extensive flexing of the run-flat tire and the large deflections associated with the deflated rolling tire, the various components within the run-flat tire undergo gradual breakdown. High component temperatures also contribute to the breakdown of the materials in the run-flat tire.
Efforts to give the crescent-shaped sidewall supporting members additional performance improvements are disclosed in U.S. Pat. Nos. 3,994,329 and 4,287,924, Japanese Patent No. 3-14370 (JP) and French Patent Nos. 1,502,689 and 2,458,407. Improvements include better heat conduction from the thickened sidewall portions, limited flexing or sagging of the deflated tire and reductions in the required thickness of the crescent-shaped reinforcement.
The chambers of the tire disclosed in U.S. Pat. No. 3,994,329 are lenticular in shape. These chambers are filled with a flexible cellular material and are bounded on both lateral sides by reinforcing layers, such as plies of the carcass. These layers form walls that render the sidewalls suitable to support the load of a wheel with limited sagging of the tire.
In U.S. Pat. No. 4,287,924 a two part crescent-shaped member has a heat conducting sheet or layer between the two parts. The layer extends over the whole height of the crescent-shaped portions and the two crescent-shaped parts are of different flexibility. The heat conducting layer may have parallel metallic cords extending radially to assist in the heat conductivity. The height of the tire is 31 percent of its inflated height when the inflation pressure is zero.
The cord reinforcing unit on the interior surface of the sidewall crescent-shaped reinforcing member in JP 3-143710 consists of at least one reinforced ply. The crescent member and the reinforced ply provides the overall sidewall support for run-flat performance. Also, the bead area has a rubber toe portion that fits into a rim recess for bead seat retention.
The FR 1,502,689 patent discloses a very thin crescent-shaped member at the interior of the sidewall which is heavily reinforced by one or two plies having reinforcing members. These reinforcing members are at an angle of .+-.30 degrees with the radial plane to help support the sidewall by triangulation with the radial reinforcing members of the carcass in the sidewall. This tire is designed for resistance of the sidewall to punctures.
A portion of the crescent-shaped reinforcing member in FR 2,458,407 is positioned inside the innerliner rubber (FIG. 3). This interior portion has some load bearing abilities, but also becomes an interior sealant material. The total thickness of the sidewall portion at an median plane of the tire is expressed as a function of the load on the tire, the section width of the tire and the radial distance from the axis of rotation to the median plane.
The features discussed above can be used in the design of a run-flat tire having some run-flat endurance capability. However, even combining all of such features will provide a run-flat tire with only limited performance capabilities. There remains a need to have improved tire performance to permit additional travel distances and especially to achieve improved load supporting capabilities for the vehicles using higher aspect ratio tires. Problems continue to inhibit run-flat tire performance when vehicle ride comfort of the inflated tire is considered. The need is to add features to create a run-flat tire which has little or no influence on the vehicle during inflated tire running but which have a significant influence after loss of tire inflation pressure, particularly in improvements to the load supporting and cornering comfort capabilities of the vehicle.
The addition of sidewall components, such as harder rubber bead fillers and reinforcing plies have been disclosed in standard T&RA tires to improve handling. However, these components in a tire degrade the ride comfort or other performance characteristics of the inflated tire.
There is a need for a new run-flat tire having improved performance characteristics that overcome some of the limitations discussed in the art. A run-flat tire that has less deflection when deflated allows the trip to continue and permits the continuation of almost normal operation of the vehicle. This is a particular need for a luxury car, family or urban-economy vehicle and the like. A durable run-flat tire is also needed that has an acceptable response to steering inputs at zero inflation pressure yet capable of adequate inflated ride comfort with a relatively soft vehicle suspension system. Furthermore, vehicles of the family or urban-economy type have available space problems as a result of their overall dimensions and the relatively large size of the passenger and luggage spaces.